4 ms·

Slightly off-topic, but: anyone round here know if there's a good reason why are batteries quoted in units of charge (mAh), instead of a unit of energy like Wh

by SEMW 13y ago

Slightly off-topic, but: anyone round here know if there's a good reason why are batteries quoted in units of charge (mAh), instead of a unit of energy like Wh or kJ? Seems like it just makes it hard to compare capacities of batteries of different voltages. A 8000mAh laptop battery sounds like it can only store three times the energy of the (2650mAh) cheap NiMH AA battery I have here, but it can actually store 30x, as the AA battery is 1.2V and assuming the laptop battery is 12V (which it may not be, I don't know).

chrissmeuk 13y ago
Yes. Amps = current. That means it can sustain 8A of current for an hour. This can be trivially divided into 4A for 2 hours, 2A for 4 hours, 1A for 8 hours etc. Also: 1. It's easy to measure electronically and calculate life from. 2. Isn't a derived non-linear figure like a Watt i.e. P=IV and voltage declines over a time on batteries making it a whole shitload of calculus and other nasty stuff. 3. Joules are hard to quantify as they are a measure of energy and not current.
fleitz 13y ago
mAh is measured at 1V at 1 Ohm. So yes, your 8000mAh stores 3X as much as the 2650 mAh battery.
- pjc50 13y ago
  No, it's measured at the battery voltage (which is usually not 1V, and varies with discharge) It's measured this way so you can most easily estimate runtime by putting an ammeter in series with the battery.
- tobiasu 13y ago
  No way. I have no idea how mAh are measured in a lab, but they are certainly not in relation to 1V. To get Wh, multiply by voltage. 2.6Ah * 1.2V = 3.1Wh. 8Ah * 4V (one cell is my assumption) = 32Wh. The battery has 10x the capacity of a NiMH (that's the lower limit, it could have two cells but that seems a bit excessive for such a device). Other than the battery discussion, Nokia devices running RT are about as interesting as the dirt on my shoes.
yread 13y ago
Because you (well, maybe not you, certainly not me, but perhaps some people) should be able to deduce the voltage from chemistry of the battery. So all LiIon LiMnO2 chemistry batteries have 3 V, LiFePO4 chemistry has 3.2 V. Then you can just multiply the voltage by mAh and you have energy in mWh
- Scene_Cast2 13y ago
  The problem with this is two-fold. First - I shouldn't have to care about battery chemistry. Second - what if they're wired up in series [1]? From personal experience, I've seen that laptop & mobile batteries list watt-hours as well. I'd prefer if that number was advertised instead. [1]With two LiPo 3.7V 3Ah cells, you can have either 7.4V 3Ah or 3.7V 6Ah.
underwater 13y ago
@chrissmeuk, you've been hellbanned.
reportingsjr 13y ago
They rate them in volts and mAh, because depending on how quickly you take energy out of the battery you can end up with more or less total battery capacity (and while the mAh rating will change as well it won't change as much as the Wh rating).

by SEMW 13y ago

chrissmeuk 13y ago

Yes. Amps = current. That means it can sustain 8A of current for an hour. This can be trivially divided into 4A for 2 hours, 2A for 4 hours, 1A for 8 hours etc. Also: 1. It's easy to measure electronically and calculate life from. 2. Isn't a derived non-linear figure like a Watt i.e. P=IV and voltage declines over a time on batteries making it a whole shitload of calculus and other nasty stuff. 3. Joules are hard to quantify as they are a measure of energy and not current.

fleitz 13y ago

mAh is measured at 1V at 1 Ohm. So yes, your 8000mAh stores 3X as much as the 2650 mAh battery.

pjc50 13y ago
No, it's measured at the battery voltage (which is usually not 1V, and varies with discharge) It's measured this way so you can most easily estimate runtime by putting an ammeter in series with the battery.
tobiasu 13y ago
No way. I have no idea how mAh are measured in a lab, but they are certainly not in relation to 1V. To get Wh, multiply by voltage. 2.6Ah * 1.2V = 3.1Wh. 8Ah * 4V (one cell is my assumption) = 32Wh. The battery has 10x the capacity of a NiMH (that's the lower limit, it could have two cells but that seems a bit excessive for such a device). Other than the battery discussion, Nokia devices running RT are about as interesting as the dirt on my shoes.

yread 13y ago

Because you (well, maybe not you, certainly not me, but perhaps some people) should be able to deduce the voltage from chemistry of the battery. So all LiIon LiMnO2 chemistry batteries have 3 V, LiFePO4 chemistry has 3.2 V. Then you can just multiply the voltage by mAh and you have energy in mWh

Scene_Cast2 13y ago
The problem with this is two-fold. First - I shouldn't have to care about battery chemistry. Second - what if they're wired up in series [1]? From personal experience, I've seen that laptop & mobile batteries list watt-hours as well. I'd prefer if that number was advertised instead. [1]With two LiPo 3.7V 3Ah cells, you can have either 7.4V 3Ah or 3.7V 6Ah.

underwater 13y ago

@chrissmeuk, you've been hellbanned.

reportingsjr 13y ago

They rate them in volts and mAh, because depending on how quickly you take energy out of the battery you can end up with more or less total battery capacity (and while the mAh rating will change as well it won't change as much as the Wh rating).